Funds are requested for a TopCount bench top microplate scintillation and luminescence counter system, configured for direct scintillation and luminescence counting of samples contained in 96-well microplates. The system will count any isotope that can be used with conventional liquid scintillation counting, as both liquid and solid scintillation counting. In addition the system will accommodate microplates from a number of different manufacturers that are designed for filter and membrane counting, adherent cell cultures, inplate immunobinding, and scintillation proximity assays. The system also includes a 96-well harvester to transfer samples from liquid suspensions onto filters for several of the above applications. Collectively, the system saves a great deal of time by eliminating the need for individual sample handling for washing samples, transfer to vials for radioactive counting, and disposal. Experience has shown that the time saved per 96-well plate is approximately 10 hours. With the use of hundreds of plates per year, this represents a remarkable increase in research efficiency. In addition to the great saving in time, the system will also greatly reduce the exposure of laboratory personnel to repetitive handling of potentially biohazardous materials as occurs with individual sample handling and produces less radioactive waste. This instrument will be housed in the Johns Hopkins University basis science core instrument facility, which has over eleven years of experience and is used by faculty throughout this institution. The initial primary users group are scientists whose research is directed to the study of viral infections that play a major role in human diseases, including human immunodeficiency virus, human papilloma virus and cervical cancer, and cytomegalovirus. It is expected that with the versatility of the instrument ant its availability in the core laboratory, the use will expand to a much greater number of investigators working with adhesion molecules, antibodies, DNA, RNA, proteins, and other experimental systems that can be adapted to the 96-well format instead of single tubes.